Dynamically reconfigurable photovoltaic system

ABSTRACT

A PV system composed of sub-arrays, each having a group of PV cells that are electrically connected to each other. A power management circuit for each sub-array has a communications interface and serves to connect or disconnect the sub-array to a programmable power grid. The power grid has bus rows and bus columns. A bus management circuit is positioned at a respective junction of a bus column and a bus row and is programmable through its communication interface to connect or disconnect a power path in the grid. As a result, selected sub-arrays are connected by selected power paths to be in parallel so as to produce a low system voltage, and, alternately in series so as to produce a high system voltage that is greater than the low voltage by at least a factor of ten.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims the benefit of the earlierfiling date of U.S. Provisional Application No. 61/695,884, filed Aug.31, 2012.

STATEMENT OF GOVERNMENT RIGHTS

This invention was developed under Contract DE-AC04-94AL85000 betweenSandia Corporation and the U.S. Department of Energy. The U.S.Government has certain rights in this invention.

FIELD

An embodiment of the invention relates to energy harvesting photovoltaic(PV) power systems such as those used in spacecraft. Other embodimentsare also described.

BACKGROUND

Energy harvesting PV power systems, also referred to as solar powersystems, have been used to provide electric power in variousapplications including residences and airborne and space borne aircraftsuch as satellites and unmanned aerial vehicles. For residentialapplications, a solar panel has a relatively small number of cells whereeach cell is quite large, such as a silicon PV cell that may be aboutsix inches by six inches in area, and there may be approximately 72 suchcells within in a single residential solar panel. Each solar celltypically is designed to produce a certain voltage, for example, about0.6 volts for silicon cells, which has only a weak dependence on theamount of light radiation received at the cell. Such cells may beelectrically connected in series within a panel, in order to increasethe harvested energy output voltage for example, 40 Volts dc (Vdc). Atypical residential solar system may include several such panels, forexample between five and ten, providing up to several hundred volts. Adc-ac conversion circuit is then used to obtain the more common 120 Vacoutput voltage.

For airborne and spacecraft applications, a PV system is used as aprimary power system that feeds energy storage devices such as abattery, as well as other components of the aircraft or spacecraft suchas the propulsion system. While the battery may have a relatively lowvoltage of less than five volts, the propulsion system may need severalhundred volts at its power supply input. Accordingly, a dc-dc upconverter or voltage boost circuit is used to increase, for example, a40-volt PV output to 800 or even 1000 volts. For space applications orunmanned aerial vehicle applications, it can be seen that a power supplybus is needed that can support low, medium and high voltages dependingupon the operational mode of the spacecraft or aircraft. For example, ahigh voltage is needed for acceleration by the propulsion unit of asatellite during orbital transfers and other maneuvers, whereas a mediumvoltage is needed for regular operations, and a low voltage is neededfor riding out a solar storm or a safe shutdown mode. In addition,reliability, availability and maintenance needs of the spacecraft oraircraft strongly impact the design of the electrical power system,which is a critical component in such applications.

SUMMARY

An embodiment of the invention is a dynamically reconfigurable energyharvesting photovoltaic (PV) system that can produce both a low voltageand alternately a high voltage, at the same harvested energy outputnode, where the high voltage may be greater than the low voltage by atleast a factor of ten. This aspect of the invention may help reduce theneed for a separate voltage boost converter, which will help improvepower efficiency in applications such as a satellite where heatdissipation may be a difficult problem. In addition, configurabilityenables a more efficient power receiver in cases where the incidentlight on the PV system is a laser beam or incoherent, not-broadbandlight beam from a remote source, as opposed to sunlight. When a lightbeam or spot “wanders” over the PV system such that a given group ofcells is not illuminated continuously, it is difficult to harvest energyefficiently. An embodiment of the invention is a PV system that canadapt itself to produce a predetermined output voltage or output powerlevel regardless of a wandering light spot.

The above summary does not include an exhaustive list of all aspects ofthe present invention. It is contemplated that the invention includesall systems and methods that can be practiced from all suitablecombinations of the various aspects summarized above, as well as thosedisclosed in the Detailed Description below and particularly pointed outin the claims filed with the application. Such combinations haveparticular advantages not specifically recited in the above summary.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention are illustrated by way of example andnot by way of limitation in the figures of the accompanying drawings inwhich like references indicate similar elements. It should be noted thatreferences to “an” or “one” embodiment of the invention in thisdisclosure are not necessarily to the same embodiment, and they mean atleast one.

FIG. 1 depicts a reconfigurable photovoltaic system.

FIG. 2 shows a more detailed view of the photovoltaic system in oneconfiguration, as part of a spacecraft application.

FIG. 3 shows the reconfigurable photovoltaic system in anotherconfiguration.

FIG. 4 shows a sub-array in which the cells are series-connected to eachother.

FIG. 5 shows a sub-array in which the cells are connected to each in acombined series-parallel fashion.

FIG. 6 depicts a sub-array power management circuit having a dc-dcconverter.

FIG. 7 depicts a cell or multi-junction power manager circuit, in blockdiagram form.

FIG. 8 depicts a photovoltaic cell with an associated cell ormulti-junction power manager circuit in greater detail.

FIG. 9 shows how a wandering laser or incoherent light beam spot coversconnected sub-arrays, while other sub-arrays outside the spot aredisconnected.

FIG. 10 shows the wandering light beam spot in a different location onthe photovoltaic system.

FIG. 11 illustrates various applications of the photovoltaic system.

DETAILED DESCRIPTION

Several embodiments of the invention with reference to the appendeddrawings are now explained. Whenever the shapes, relative positions andother aspects of the parts described below are not clearly defined, thescope of the invention is not limited only to the parts shown, which aremeant merely for the purpose of illustration. Also, while numerousdetails are set forth, it is understood that some embodiments of theinvention may be practiced without these details. In other instances,well-known circuits, structures, and techniques have not been shown indetail so as not to obscure the understanding of this description.

FIG. 1 depicts a reconfigurable PV system 1 in accordance with anembodiment of the invention. The system is composed of a number of PVenergy harvesting sub-arrays 2. Although only four are shown, the systemis of course not limited to that number as there may be as few as twosub-arrays 2 or there can be greater than four. Each sub-array 2contains a group of PV cells 3 that are electrically connected to eachother in series for a desired higher output voltage (see FIG. 4), inparallel for a desired increased current, or in a series-parallelcombination to yield both higher output voltage and higher current (seeFIG. 5). Mixed or non-symmetrical arrangements of parallel connectedseries strings of cells are also possible. The cell 3 may be amicrosystem enabled photovoltaic (MEPV) cell that can be manufacturedusing semiconductor microelectronic fabrication techniques and be mayrelatively small, e.g., between 100 microns and 5 mm in diameter, and aslow as 1 micron in thickness such as in a III-V semiconductor cell.Given the small size of the MEPV cell, the sub-array 2 can havethousands of cells 3 (in contrast to the 72 cells in a conventional PVmodule). Note also that not all of the cells 3 in a sub-array 3 need bereplicates or even of the same type. For example, some may be siliconothers may be Ge or III-V cells. The cell 3 may alternatively be amulti-junction cell that has a combination of two or more junctions thatmay be connected in series or, as described below in accordance with anembodiment of the invention depicted in FIGS. 7-8. For example, each ofthe sub-arrays 2 may be composed of photocells each of which has anactive or light detection area that is less than five (5) squaremillimeters in area, and wherein each of the sub-arrays 2 may haveseveral thousand of MEPV photocells and may produce between 1 volt(e.g., two Si silicon cells in series) and 1000 volts dc, with currentsin the range 1 uAmpere to several Amperes, uAmperes for applicationswhere only high voltage and low or essentially no current is needed(electron/ion acceleration grids, for example), and several Amperes forhigh current draw applications (e.g., thermal loads, etc.) it would bepossible to transfer mW to kW of power using the power transferconfiguration described here.

Still referring to FIG. 1, in the presence of incident light, asub-array output voltage is produced by each sub-array 2, from itsconnected cells 3, at a respective pair of sub-array power nodes. Theseare sometimes designated with the labels (+) and (−) to indicate thepolarity of the output voltage. The output voltage and current or outputpower of the sub-array is delivered by a distributed network ofconductors and active circuits referred to here as a power grid. Thepower grid (or power bus interconnect) is composed of multiple bus rowsand multiple bus columns. As can be seen, a bus management circuit 5 ispositioned at a respective junction of a bus column and a bus row. Eachbus row has a respective number of bus group row segments 6 that arecoupled in a daisy chain manner, or forming a sequence, by some of thebus management circuits 5. Similarly, each bus column has a respectivenumber of bus group column segments 7 that are also coupled in a daisychain manner, by some of the bus management circuits 5. Each bus groupsegment (row segment 6 or column segment 7) has a respective number ofbus conductors. In one example, each bus group segment has two busconductors, as seen in FIG. 2 and in FIG. 3, although additionalconductors may be added in parallel, for example, to reduce electricalresistance. In such a power grid, each bus management circuit 5 may becoupled to between two and four adjacent bus group segments, namely leftand right bus group row segments 6, and upper and lower bus groupcolumns segments 7.

Coupled to each pair of sub-array output power nodes is the input of arespective sub-array power management circuit 4. The circuit 4 also hasa power output that is coupled to the power grid, i.e., to either a rowsegment 6 or a column segment 7. In one embodiment, the current pathswitches in each sub-array power management circuit 4 support a “meshnetwork” in that they can connect any of the input nodes of the circuit4 with any of its output nodes. The circuit 4 also has a communicationsinterface, which is not shown in FIG. 1 but can be seen in FIG. 2 whereit is coupled to a communications grid 9. The communications grid 9 towhich the communications interfaces of the power management circuits 4and bus management circuits 5 are coupled may be any suitable, relativelow complexity and low bit rate digital communications bus. Thiscommunication interface could be optical in nature with the informationpassed to the power management circuit through a signal encoded on thelight that is illuminating the sub-arrays and that is decoded by acommunications band decoder—see FIG. 2 described below.

The sub-array power management circuit 4 contains circuitry includingsolid state current path switches, switch drivers, control logic, andcommunications interface circuitry that enables it to be programmable(through its communication interface) during in-the-field use of the PVsystem, to either connect or disconnect its respective sub-array 2 tothe power grid. In addition to the programmable sub-array powermanagement circuits 4, each of the bus management circuits 5 is alsoprogrammable (through its communication interface) to one of connect anddisconnect a power or current path in the power grid, using internalcurrent path switches that may also support a mesh network (similar tothe capability of the circuit 4 described above). Those two capabilitiestogether enable two or more selected sub-arrays 2 to be connected,through selected current or power paths in a “programmable” power grid,in parallel so as to produce a low voltage at the harvested energyoutput node but at a high current. Alternately, the configurability ofthe sub-array power management circuits 4 and the bus managementcircuits 5 enable two or more selected sub-arrays 2 to be connected, viaselected current paths, in series so as to produce a higher voltage thatmay be greater than the lower voltage by at least a factor of ten(depending upon a sufficient number of sub-arrays 2 being available fora series connection).

Referring now to FIG. 2 for additional details concerning the power gridand the sub-array power management circuits 4, FIG. 2 shows an examplepower path that has been created in the power grid, by appropriatelyprogramming the current path switches in selected circuits 4, 5. Theexample power path enables a series connection of sub-arrays 2_1, 2_2, .. . which may lie in the same column, such that the (+) harvested energyoutput node is a conductor of a bus row segment 6 at a top boundary ofthe PV system, while the (−) harvested energy output node is a conductorof a bus row segment 6 at a bottom boundary of the PV system. A busmanagement circuit 5 that is not at the boundary of the PV system can beprogrammed to alternately connect and disconnect to each other a) a busconductor from any one of its four adjacent bus group segments and b) abus conductor from any other one of its four adjacent bus groupsegments, providing maximum flexibility in defining a power path.

For the example of FIG. 2, it can be seen that additional columns ofsub-arrays can be connected to each other in series similar to the onein FIG. 2. If these additional series-connected columns happen to beadjacent to another, then all of these columns can also be connected toeach other in parallel, along the top and bottom rows of the PV system,by programming the bus management circuit 5 that is at the boundary, tocreate a further current path from its left bus row segment 6 to itslower bus column segment 7.

The power grid of FIG. 2 can be reconfigured so that power paths arecreated therein that enable a parallel, rather than a series, connectionof the sub-arrays 2_1, 2_2, . . . This is depicted in the example ofFIG. 3. If additional current is desired, then additionalparallel-connected columns of sub-arrays can be created, and these maybe placed in parallel with each other by suitably programming the busmanagement circuits 5 at the top and bottom boundaries of the PV system.

It should be noted that while the full mesh capability of the internalcurrent path switches of the circuits 4, 5 described above can providethe greatest flexibility in creating power paths in the power gird andbetween sub-arrays, an alternative is to restrict the number of currentpath switches so that the circuit 4 or 5 has less than a full meshcapability. This may be acceptable so long as the desired power pathscan be created in the power grid, and a desired level of granularity ofthe configurability of the PV system as a whole, including granularityof the connections between sub-arrays, can be met.

Still referring to FIG. 2, in accordance with another embodiment of theinvention, the remote light beam source can encode power transferconfiguration instructions or other data (e.g., instructions forcontrolling the power system configuration or other sub-system such asthe ADCS, COM, etc.) by imposing a high frequency component onto thelight beam. That information is then detected through an electrical nodethat is capacitively coupled to the PV cells, while the DC component isbeing harvested for power. An example is shown in FIG. 2 where the acsignal can be out-coupled through a series capacitor connected to theharvested energy output node, to a communications band decoder. Thelatter translates or decodes the information or data signal for use bythe EPS controller for example, to use in configuring the powermanagement circuits and/or determining for example that the incidentlight is a remotely sourced light beam, rather than sunlight, and/or forcontrolling other sub-systems.

Turning now to FIG. 6, a sub-array power management circuit 4 isdepicted that has a dc-dc converter coupled between the sub-array outputnodes (of its associated sub-array 2) and the power grid. Note that inthis particular example, the current path switches (that serve toalternately connect and disconnect the sub-array 2 to the power grid)are “in front” of the dc-dc converter circuitry, so that when thesub-array 2 is disconnected from the grid, the dc-dc converterautomatically sees essentially Volts at its input. While it is possibleto allow the dc-dc converter to also be controlled through thecommunications grid 9 (as shown here via the communication interface ofthe circuit 4), for example as to how much to boost the output voltageor at what voltage to regulate, this is not needed in all instancesbecause a fixed boost or regulated voltage may be set. Using the dc-dcconverter as a boost converter, the PV system can provide a boostedoutput voltage at its harvested energy output nodes, whileadvantageously distributing the task of dissipating the heat produced bythe overall boost conversion process, to the various locations of thesub-arrays (rather than to a centralized location outside the PVsystem). The use of a dc-dc converter in this manner may be viewed as analternative to the embodiment described above where a sufficient numberof sub-arrays 2 are connected in series (by appropriately configuringthe power paths in the power grid) in order to produce a high outputvoltage, or it may be used in conjunction with the series configurationof FIG. 2, for example, so as to obtain the highest available voltagefrom the PV system (as the sum of the output voltages of a number ofsub-array power management circuits 4 that also have boost converters asdepicted in FIG. 6).

Returning briefly to FIG. 2, this figure also serves to illustrateanother embodiment of the invention, namely a specific application orintegration of a dynamically reconfigurable PV system in a spacecraft.In such an application, the spacecraft has an electrical power system(EPS), which includes (in addition to the PV system) a controller 8 anda power distribution network 10. The EPS distributes power from theharvested energy output nodes of the PV system to other components ofthe spacecraft, namely a rechargeable battery, an on-board computer(OBC), a communications subsystem (COM), and an attitude determinationand control system (ACDS). As suggested above, this may call for low,medium and high voltages to be alternately available on the harvestedenergy output nodes dynamically, i.e. during normal use or deployment ofthe satellite. The reconfigurable PV system described above may meetsuch requirements, when the EPS controller 8 has been programmed toconfigure the power and bus management circuits 4,5 via thecommunications grid 9, to set the high or low voltage at the harvestedenergy output node.

Turning now to FIG. 7, this figure depicts an embodiment of theinvention where the PV cell 3 is a multi-junction cell, and a cell-levelor multi-junction power manager circuit 14 is provided for each suchcell, to produce the cell output voltage. As was suggested earlier, oneor more of the cells in each sub-array 2 may be multi-junction cells,wherein each multi-junction cell has two or more junctions (e.g., p-njunctions) for collecting the photo-generated current, and wherein eachof the junctions is independently coupled to the multi-junction powermanager circuit 14. The different junctions (the example here referringto three junctions A, B, and C) may each be tuned or optimized toproduce the most electricity when absorbing a different color orwavelength of light. Each of the photocell junctions that make up amulti-junction PV cell is coupled to a separate input port of the powermanager circuit 14 associated with that multi-junction PV cell. As such,the energy harvested by every one of the photocell junctions is providedto the power grid through the same output port of the power managementcircuit 14. The power manager circuit 14 which is managing the powercoming from the different junctions within the multi-junction cellscould manage power coming from a single junction or an aggregate ofsimilar junctions, within a sub-array of multi-junction cells. It alsoshould be noted that a single element within a multi-junction cell couldbe either a single junction or perhaps a subset or multiple being two(or more) junctions connected, for example, in series to make oneelement of a larger, for example, six-junction multi-junction cell. Asanother example of a multi-junction cell, consider one that has a totalof five junctions, where various subsets of the junctions areindependently coupled to a multi-junction power manager circuit 14. Forinstance, such a 5-junction cell can be arranged as follows: a2-junction sub-cell connected separately (to the circuit 14) from a 1junction sub-cell and another 2-junction sub-cell. Other arrangements ofmulti-junction cells that may have series and/or parallel connectedsubsets, i.e. connected to its respective power manager circuit 14, arepossible.

In one embodiment, referring now to FIG. 8, the power manager circuit 14may be composed of a power detection circuit that serves to detect somemeasure of the relative power being produced at any given time by thejunctions A, B, and C. For example, the detector may be designed toautomatically detect which one or more of the junctions are producingthe lowest power. In response to such a determination, the PV cell 3,and in particular its power manager circuit 14, will operate in apredetermined mode. As an example, that mode may be one where controlsignals are asserted to configure the current path switches so that thejunctions producing the lowest power become disconnected from the celloutput port.

In another example, the detector may be designed to automatically detectwhich one or more of the junctions are producing the highest power, inresponse to which the cell 3 will operate in a different predeterminedmode. As an example, that mode may be one where control signals areasserted to configure the current path switches so that only thejunctions producing the highest power become connected in series withthe cell output port. In another embodiment, the cell or multi-junctionpower manager circuit 14 has a communication interface through which itcan be programmed (via the communication grid 9) so as to connect thephotocell junctions, which make up its associated multi-junction PV cellor group of such multi-junction PV cells, either a) all in parallel witheach other, b) all in series with each other, or c) in someseries-parallel combination.

It should be noted that the cell or multi-junction power manager circuit14 and its associated multi-junction PV cell 3 could be implemented onthe same microelectronic or integrated circuit substrate.

Referring now to FIG. 9 and to FIG. 10, these figures are used toillustrate another embodiment of the invention, where the PV system canmaintain a predetermined system output voltage or system output powerlevel despite the presence of a wandering laser light beam or incoherentlight beam illumination spot. A similarly beneficial result may beobtained in situations where there is a partial shading of thesub-arrays 2. The PV system depicted here is being illuminated with alaser or incoherent beam (not sunlight), or can be viewed as beingshaded outside the illumination spot. Examples of these circumstancesare given in FIG. 11 where remote power transfer is occurring to a powerreceiver or PV system of an aircraft or spacecraft, via a remotelyproduced light beam that is being aimed at the sub-arrays 2. To maintainefficiency, the beam spot should be no larger than the area of thesub-arrays 2 of the PV system. In fact, the spot should be smaller thanthe full area of coverage of the sub-arrays 2 (as shown by the example)to allow for adequate misalignment tolerance between the remote lightbeam source and the PV array. In traditional PV systems, illuminatingless than the full area of the PV array leads to reduced performance andpossibly damage to the array. However, an embodiment of the inventioncan accept a laser (or other light) spot size that is less than the fullarea of the PV array. This illumination situation will yield some lowperforming sub-arrays 2 outside the spot, and some high performingsub-arrays 2 inside the spot. Now, the EPS controller 8 programs thepower manager circuits 14 of the low performing sub-arrays to disconnectthem from the power grid, in response to, for example, a signal from thepower detector (see FIG. 8) that is associated with a multi-junctioncell in each of those sub-arrays that indicates low performance by amulti-junction cell in the sub-array. Alternatively, the power managercircuit 14 could have a DC-DC voltage boost converter that allows thepartially illuminated or low performing sub-arrays to connect with thepower grid, at the appropriate voltage. The power situation would needto be one where the power output from that circuit 14 doesn't need tomatch the grid power, but its voltage output level does need to match,for the sub-array to be connected to the power grid. In addition, theEPS controller programs the power manager circuits 14 of high performingsub-arrays to connect them to the power grid, in response to a signalfrom the power detector associated with a multi-junction cell in each ofthose sub-arrays that indicates high performance by a multi-junctioncell in the sub-array.

In another embodiment, the EPS controller 8 signals each of the powermanager circuits 14 to connect their photocell junctions in series witheach other in response to a determination that the PV system is mostlikely feeing sufficient sunlight, so that efficient energy harvestingcan be performed when the current characteristics of the differentjunctions sufficiently match during sunlight. But when it is determinedthat energy harvesting is based on a laser light beam or an incoherentlight beam (not sunlight), then the process described below can beperformed to disconnect the junctions that are not optimized for thecolor or wavelength of the light beam, and also to track the wanderingbeam spot so that an optimal selection of a subset of the sub-arrays isalways being made (consistent with the coverage area of the beam spot).

A method for operating an energy harvesting photovoltaic (PV) systemhaving energy harvesting sub-arrays, cell power management circuits,sub-array power management circuits, and a programmable power grid, mayproceed as follows (with references being made also to FIG. 9 and FIG.10). A number of performance indications are received (e.g., by the EPScontroller 8) from certain cell power manager circuits 14, respectively(see also FIG. 7 and FIG. 8). The controller 8 then signals the powermanager circuits 14 of low performing sub-arrays to disconnect thosesub-arrays from the power grid. In addition, the controller signals thecircuits 14 of high performing sub-arrays to connect those sub-arrays tothe power grid. Also, the controller 8 signals the bus managementcircuits 5 (see FIG. 1, and also FIG. 2 and FIG. 3) to form power pathsin the power grid, from the connected sub-arrays to the pair ofharvested energy output nodes of the PV system 1, based on the receivedperformance indications. This may be designed to achieve a predeterminedsystem output voltage or system output power level.

Next, while the beam spot wanders over the PV system 1, the operationsin the previous paragraph are automatically being repeated by thecontroller 8 upon the power grid so as to strive to maintain thepredetermined system output voltage or power level, by making changes orupdates such that only well-illuminated sub-arrays remain connected tothe power grid.

In the description above, for the purposes of explanation, numerousspecific details have been set forth in order to provide a thoroughunderstanding of the embodiments. It will be apparent however, to oneskilled in the art, that one or more other embodiments may be practicedwithout some of these specific details. The particular embodimentsdescribed are not provided to limit the invention but to illustrate it.The scope of the invention is not to be determined by the specificexamples provided above but only by the claims below. For example,although in FIG. 2 and in FIG. 3 the harvested energy output nodes areselected to be located at the top and bottom boundaries of the PVsystem's power grid, they could alternatively be located at the left andright boundaries. In other instances, well-known structures, devices,and operations have been shown in block diagram form or without detailin order to avoid obscuring the understanding of the description. Whereconsidered appropriate, reference numerals or terminal portions ofreference numerals have been repeated in the figure to indicatecorresponding or analogous elements, which may optionally have similarcharacteristics.

It should also be appreciated that reference throughout thisspecification to “one embodiment”, “an embodiment”, “one or moreembodiments”, or “different embodiments”, for example, means that aparticular feature may be included in the practice of the invention.Similarly, it should be appreciated that in the description, variousfeatures are sometimes grouped together in a single embodiment, figure,or description thereof for the purpose of streamlining the disclosureand aiding in the understanding of various inventive aspects. Thismethod of disclosure, however, is not to be interpreted as reflecting anintention that the invention requires more features than are expresslyrecited in each claim. Rather, as the following claims reflect,inventive aspects may lie in less than all features of a singledisclosed embodiment. Thus, the claims following the DetailedDescription are hereby expressly incorporated into this DetailedDescription, with each claim standing on its own as a separateembodiment of the invention.

We claim:
 1. A dynamically reconfigurable energy harvesting photovoltaic(PV) system comprising: a plurality of PV energy harvesting sub-arrayswherein each sub-array comprises a group of photovoltaic cells that areelectrically connected to each other to generate a voltage at arespective one of a plurality of pairs of sub-array power nodes; aplurality of power management circuits each having a power input that iscoupled to a respective one of the pairs of sub-array power nodes and acommunications interface; and a programmable power grid to which a poweroutput of each of the power management circuits is coupled, the powergrid having a plurality of bus rows, a plurality of bus columns, aplurality of bus management circuits each being positioned at arespective junction of a bus column and a bus row, and a harvestedenergy output node, wherein each power management circuit isprogrammable through its communication interface during in-the-field useof the PV system to one of connect and disconnect its respectivesub-array to the grid, and each bus management circuit is programmablethrough its communication interface to one of connect and disconnect apower path in the grid, so that selected sub-arrays are connected byselected power paths to be in parallel so as to produce a low voltage atthe harvested energy output node, and, alternately, selected sub-arraysof the system are connected by selected power paths to be in series soas to produce a high voltage that is greater than the low voltage by atleast a factor of ten.
 2. The system of claim 1 wherein each bus row hasa respective plurality of bus group segments that are coupled in a daisychain manner by some of the bus management circuits, and wherein eachbus column has a respective plurality of bus group segments that arecoupled in a daisy chain manner by some of the bus management circuits,each bus group segment having a respective plurality of bus conductors.3. The system of claim 2 wherein each bus management circuit is coupledto four adjacent bus group segments, and can be programmed toalternately connect and disconnect to each other a) a bus conductor fromone of the four adjacent bus group segments and b) a bus conductor fromanother one of the four adjacent bus group segments.
 4. The system ofclaim 1 wherein each of the power management circuits has a dc-dcconverter coupled between the power input of the bus management circuitand the power grid.
 5. The system of claim 1 wherein some of the cellsin each sub-array are multi-junction cells, wherein each multi-junctioncell has a plurality of junctions, wherein each of the junctions, or atleast one subset of said junctions is independently coupled to amulti-junction power manager circuit associated with the multi-junctioncell, the system further comprising an electrical power systemcontroller that is to program the power management circuits of lowperforming sub-arrays to one of a) disconnect the sub-arrays from thepower grid in response to a signal from a multi-junction power managercircuit associated with a multi-junction cell in each of the sub-arraysthat indicates low performance of a multi-junction cell in thesub-array, and b) where the multi-junction power manager circuitcomprises a dc-dc boost voltage converter, connect the low-performingsub-arrays to the power grid through the boost voltage converter,wherein the controller is to program the power management circuits ofhigh performing sub-arrays to connect the sub-arrays to the power gridin response to a signal from a multi-junction power management circuitassociated with a multi-junction cell in each of the sub-arrays thatindicates high performance of a multi-junction cell in the sub-array. 6.The system of claim 1 further comprising: a communications grid to whichthe communications interfaces of the power management circuits and busmanagement circuits are coupled; and an electrical power systemcontroller that is to program the power and bus management circuits, viathe communications grid, to set the high or low voltage at the harvestedenergy output node.
 7. The system of claim 1 wherein each of thesub-arrays is composed of photocells each of which has an active orlight detection area that is less than live (5) square millimeters inarea, and wherein each of the sub-arrays has at least one thousandphotocells and is to produce between 1 volt dc and 1000 volts dc.
 8. Thesystem of claim 1 further comprising a communications band decoder thatis capacitively coupled to a sub-array power node, to decode informationor data from a signal detected by the sub-array, wherein saidinformation or data was embedded by a remote source of light that isilluminating the sub-array.
 9. A dynamically reconfigurable energyharvesting photovoltaic (PV) system comprising: a plurality of PV energyharvesting sub-arrays wherein each sub-array comprises a group ofmulti-junction PV cells each of which has a plurality of photocelljunctions that are optimized to produce electricity by absorbing aplurality of different wavelengths of light, and a plurality of powermanagement circuits each being associated with and coupled to arespective one or multiples of the multi-junction PV cells; and a powergrid to which a power output of each of the power management circuits iscoupled, wherein each or multiple subsets of the plurality of photocelljunctions that make up a multi-junction PV cell is coupled to a separateInput port of the power management circuit associated with thatmulti-junction PV cell or plurality of multi-junction cells, wherein allof the energy harvested by all of the photocell junctions that make upthe associated multi-junction PV cell or cells is provided to the gridthrough the output port of the power management circuit.
 10. The systemof claim 9 wherein the power management circuit is to automaticallydetect which one or more of the photocell junctions that make up theassociated multi-junction PV cell is producing the lowest power and inresponse operate in a predetermined mode.
 11. The system of claim 10wherein in said predetermined mode, the photocell junctions producingthe lowest power become disconnected from the output port.
 12. Thesystem of claim 9 wherein the power management circuit is toautomatically detect which one or more of the photocell junctions thatmake up the associated multi-junction PV cell is producing the highestpower and in response operate in a predetermined mode.
 13. The system ofclaim 12 wherein in said predetermined mode, only the photocelljunctions producing the highest power become connected in series withthe output port.
 14. The system of claim 9 wherein each of the powermanagement circuits comprises a communication interface through which itcan be programmed so as to connect the photocell junctions, which makeup its associated multi-junction PV cell, one of a) all in parallel witheach other, b) all in series with each other, or c) a combination ofparallel, and series connections.
 15. The system of claim 14 furthercomprising an electrical power system controller that is to signal eachof the power management circuits to connect their photocell junctions inseries with each other in response to a determination that energyharvesting is based on sunlight incident upon the multi-junction PVcells.
 16. The system of claim 14 further comprising an electrical powersystem controller that is to signal a selected subset of the powermanagement circuits to operate in a predetermined mode in response to adetermination that energy harvesting is based on a light beam such aslaser light or an incoherent light beam, rather than sunlight, that isincident primarily upon the multi-junction PV cells associated with theselected subset.
 17. A method for operating an energy harvestingphotovoltaic (PV) system having energy harvesting sub-arrays, cell powermanagement circuits, sub-array power management circuits, and aprogrammable power grid, comprising: a) receiving a plurality ofperformance indications from a plurality of cell power managementcircuits, respectively, that are associated with a plurality ofsub-arrays in the system, respectively, wherein each sub-array has apair of power output nodes that are coupled to a respective sub-arraypower management circuit; b) signaling the power management circuits oflow performing sub-arrays to disconnect those sub-arrays from a powergrid; c) signaling the power management circuits of high performingsub-arrays to connect those sub-arrays to the power grid; d) signalingthe bus management circuits to form current paths from the connectedsub-arrays to a pair of harvested energy output nodes of the systembased on the received performance indications and to maintain apredetermined system output voltage or system output power level,repeating a)-d) so as to maintain the predetermined system outputvoltage or power level in the presence of changing illumination whichresults in changes to which of the sub-arrays are connected to the powergrid, as per b) and c).
 18. The method of claim 17 wherein the receivingof performance indications from the cell power management circuits, andsignaling of the power and bus management circuits is performed by asatellite or aircraft EPS controller.
 19. The method of claim 17 whereinat least one of the sub-arrays has multi-junction PV cells, the methodfurther comprising the cell power management circuits associated with amulti-junction PV cell self configuring its current path switches basedon having detected relative power or current levels of the individualjunctions in the multi-junction PV cell.